SỔ TAY ĐIỀU CHỈNH LA BÀN TỪ (HANDBOOK OF MAGNETIC COMPASS)

HANDBOOK OF MAGNETIC
COMPASS ADJUSTMENT

NATIONAL
GEOSPATIAL-INTELLIGENCE AGENCY
BETHESDA, MD
2004

(Formerly Pub.No. 226)
AS ORIGINALLY PUBLISHED BY

DEFENSE MAPPING AGENCY
HYDROGRAPHIC/TOPOGRAPHIC CENTER
WASHINGTON, D.C.
1980


INTRODUCTION
This document has been prepared in order to present all pertinent information regarding the practical procedures of
magnetic compass adjustment in one text. As such, it treats of the basic principles of compass deviations and their correction,
and not of the details of particular compass equipment.
Although this text is presented as a systematic treatise on compass adjustment, ship's personnel who are inexperienced with
compass correction will find sufficient information in Chapters I and XIV to eliminate compass errors satisfactorily without
intensive study of the entire text. Reference should also be made to figure 318 for condensed information regarding the
various compass errors and their correction.
In this handbook, the term compass adjustment refers to any changes of permanent magnet of soft iron correctors whereby
normal compass errors are reduced. The term compass compensation refers to any change in the current supplied to compass
compensating coils whereby the errors due to degaussing are reduced.
The basic text is the outgrowth of lecture notes prepared by Nye S. Spencer and George F. Kucera while presenting courses
of instruction in adjustment and compensation during World War II at the Magnetic Compass Demonstration Station, Naval
Operating Base, Norfolk, Virginia.

CHAPTER I
PROCEDURES FOR MAGNETIC COMPASS ADJUSTMENT (CHECK-OFF LIST)
NOTE: If the magnetic adjustment necessitates (a) movement of degaussing compensating coils, or (b) a change of Flinders
bar length, the coil compensation must be checked. Refer to Chapter XIV.
101. Dockside tests and adjustments
1. Physical checks on the compass and binnacle.
(a) Remove any bubbles in compass bowl (article 402).
(b) Test for moment and sensibility of compass needles (article 403).
(c) Remove any slack in gimbal arrangement.
(d) Magnetization check of spheres and Flinders bar (article 404).
(e) Alignment of compass with fore-and-aft line of ship (article 405).
(f) Alignment of magnets in binnacle.
(g) Alignment of heeling magnet tube under pivot point of compass.
(h) See that corrector magnets are available.
2. Physical checks of gyro, azimuth circle, and peloruses.
(a) Alignment of all gyro repeater peloruses or dial peloruses with fore-and-aft line of ship (article 405).
(b) Synchronize gyro repeaters with master gyro.
(c) Make sure azimuth circle and peloruses are in good operating condition.*
3. Necessary data.
(a) Past history or log data which might establish length of Flinders bar (articles 407 and 607)
(b) Azimuths for given date and observer's position (Chapter VIII).
(c) Ranges or distant objects in vicinity (local charts).*
(d) Correct variation (local charts).
(e) Degaussing coil current settings for swing for residual deviations after adjustment and compensation (ship's
Degaussing Folder).
4. Precautions.
(a) Determine transient deviations of compass from gyro repeaters, doors, guns, etc. (Chapter X).
(b) Secure all effective magnetic gear in normal seagoing position before beginning adjustments.
(c) Make sure degaussing coils are secured before beginning adjustments. Use reversal sequence, if necessary.

(d) Whenever possible, correctors should be placed symmetrically with respect to the compass (articles 318
and 613).
5. Adjustments.
(a) Place Flinders bar according to best available information (articles 407, 608 and 609).
(b) Set spheres at midposition, or as indicated by last deviation table.
(c) Adjust heeling magnet, using balanced dip needle if available (Chapter XI).
*

Applies when system other than gyro is used as heading reference

1


102. Adjustments at sea. These adjustments are made with the ship on an even keel and after steadying on each heading.
When using the gyro, swing from heading to heading slowly and check gyro error by sun's azimuth or ranges on each heading
if desired to ensure a greater degree of accuracy (article 706). Be sure gyro is set for the mean speed and latitude of the
vessel. Note all precautions in article 101(4) above. "OSCAR QUEBEC" international code signal should be flown to
indicate such work is in progress. Chapter VII discusses methods for placing the ship on desired headings.
1. Adjust the heeling magnet, while the ship is rolling, on north and south magnetic heading until the oscillations
of the compass card have been reduced to an average minimum. (This step is not required if prior adjustment
has been made using a dip needle to indicate proper placement of the heeling magnet.)
2. Come to an east (090°) cardinal magnetic heading. Insert fore-and-aft B magnets, or move the existing B
magnets, in such a manner as to remove all deviation.
3. Come to a south (180°) magnetic heading. Insert athwartship C magnets, or move the existing C magnets, in
such a manner as to remove all deviation.
4. Come to a west (270°) magnetic heading. Correct half of any observed deviation by moving the B magnets.
5. Come to a north (000°) magnetic heading. Correct half of any observed deviation by moving the C magnets.
(The cardinal heading adjustments should now be complete.)
6. Come to any intercardinal magnetic heading, e.g. northeast (045°). Correct any observed deviation by moving
the spheres in or out.

7. Come to the next intercardinal magnetic heading, e.g. southeast (135°). Correct half of any observed deviation
by moving the spheres. (The intercardinal heading adjustments should now be complete, although more
accurate results might be obtained by correcting the D error determined from the deviations on all four
intercardinal heading, as discussed in article 501.)
8. Secure all correctors before swinging for residual deviations.
9. Swing for residual undegaussed deviations on as many headings as desired, although the eight cardinal and
intercardinal headings should be sufficient.
10. Should there still be any large deviations, analyze the deviation curve to determine the necessary corrections
and repeat as necessary steps 1 through 9 above (Chapter V).
11. Record deviations and the details of corrector positions on standard Navy Form NAVSEA 3120/4 and in the
Magnetic Compass Record NAVSEA 3120/3 (article 901).
12. Swing for residual degaussed deviations with the degaussing circuits properly energized (Chapter XIV).
13. Record deviations for degaussed conditions on standard Navy Form NAVSEA 3120/4.
103. The above check-off list describes a simplified method of adjusting compasses, designed to serve as a simple workable
outline for the novice who chooses to follow a step-by-step procedure. The "Dockside Tests and Adjustments" are essential
as a foundation for the "Adjustments at Sea", and if neglected may lead to spurious results or needless repetition of the
procedure at sea. Hence, it is strongly recommended that careful considerations be given these dockside checks prior to
making the final adjustment so as to allow time to repair or replace faulty compasses, anneal or replace magnetized spheres or
Flinders bar, realign binnacle, move gyro repeater if it is affecting the compass, or to make any other necessary preliminary
repairs. It is further stressed that expeditious compass adjustment is dependent upon the application of the various correctors
in a logical sequence so as to achieve the final adjustment with a minimum number of steps. This sequence is incorporated in
the above check-off list and better results will be obtained if it is adhered to closely. Figure 318 presents the various compass
errors and their correction in condensed form. The table in figure 103 will further clarify the mechanics of placing the
corrector magnets, spheres, and Flinders bar. Chapter IV discusses the more efficient and scientific methods of adjusting
compasses, in addition to a more elaborate treatment of the items mentioned in the check-off list. Frequent, careful
observations should be made to determine the constancy of deviations and results should be systematically recorded.
Significant changes in deviation will indicate the need for readjustment.
To avoid Gaussin error (article 1003) when adjusting and swinging ship for residuals, the ship should be steady on the
desired heading for at least 2 minutes prior to observing the deviation.

2


Fore-and-aft and athwartship magnets

â

W on NE'ly,
E on SE'ly,
W on SW'ly,
and
E on NW'ly.
(– D error)

Place magnets red
aft.

No spheres on
binnacle.

Place spheres
athwartship.

Place spheres fore
and aft.

Raise magnets.

Lower magnets.

Spheres at
athwartship
position.

Move spheres
toward compass or
use larger spheres.

Lower magnets.

Raise magnets.

Spheres at fore
and aft position.

Easterly on north
and westerly on
south.

Westerly on north
and easterly on
south.

(+ C error)

(– C error)

Place athwartship
magnets red
starboard

Place athwartship
magnets red port

E on E'ly and W
on W'ly when sailing toward equator
from N latitude or
away from equator
to S latitude

W on E'ly and E
on W'ly when sailing toward equator
from N latitude or
away from equator
to S latitude

No bar in holder.

Place required
amount of bar
forward.

Place required
amount of bar aft.

Move spheres
outward or
remove.

Bar forward of

binnacle.

Increase amount
of bar forward.

Decrease amount
of bar forward.

Move spheres
outward or
remove.

Move spheres
toward compass or
use larger spheres.

Bar aft of
binnacle.

Decrease amount
of bar forward.

Increase amount
of bar forward.

E on N'ly,
W on E'ly,
E on S'ly,
and
W on W'ly

(+ E error)

W on N'ly,
E on E'ly,
W on S'ly,
and
E on W'ly
(– E error)

á
Bar

W on E'ly and E
on W'ly when sailing toward equator
from S latitude or
away from equator
to N latitude

E on E'ly and W
on W'ly when sailing toward equator
from S latitude or
away from equator
to N latitude

No spheres on
binnacle

Place spheres at
port forward and
starboard aft intercardinal positions.

Place spheres at
starboard forward
and port aft intercardinal positions.

Lower magnets

Spheres at
athwartship
position

Slew spheres
clockwise through
required angle.

Slew spheres
counter-clockwise
through required
angle.

If compass north is attracted to high side of ship when rolling,
raise the heeling magnet if red end is up or lower the heeling
magnet if blue end is up.

Raise magnets

Spheres at fore
and aft position

Slew spheres

counter-clockwise
through required
angle.

Slew spheres
clockwise through
required angle.

If compass north is attracted to low side of ship when rolling,
lower the heeling magnet if red end is up or raise the heeling
magnet if blue end is up.
NOTE: Any change in placement of the heeling magnet will
affect the deviations on all headings.

Westerly on east
and easterly on
west.

(+ B error)

(– B error)

No fore and aft
magnets in
binnacle.

Place magnets red
forward.

Fore and aft

magnets red
forward.
Fore and aft
magnets red aft.

Magnets
â

Deviation à
Magnets
â
No athwartship
magnets in
binnacle
Athwartship
magnets red
starboard

Athwartship
magnets red port

Flinders bar

E on NE'ly,
W on SE'ly,
E on SW'ly,
and
W on NW'ly.
(+ D error)

Easterly on east
and westerly on
west.

Deviation à

Quadrantal spheres

Raise magnets

Lower magnets

Deviation à
Spheres

Deviation à
Spheres
â

Deviation change
with change in
latitude à
Bar
â

Deviation change
with change in
latitude à

Figure 103 – Mechanics of magnetic compass adjustment

3

Heeling magnet
(Adjust with changes in magnetic latitude)


CHAPTER II
MAGNETISM
201. The magnetic compass. The principle of the present day magnetic compass is in no way different from that of the
compass used by the ancients. It consists of a magnetized needle, or array of needles, pivoted so that rotation is in a
horizontal plane. The superiority of the present day compass results from a better knowledge of the laws of magnetism,
which govern the behavior of the compass, and from greater precision in construction.
202. Magnetism. Any piece of metal on becoming magnetized, that is, acquiring the property of attracting small particles of
iron or steel, will assume regions of concentrated magnetism, called poles. Any such magnet will have at least two poles, of
unlike polarity. Magnetic lines of force (flux) connect one pole of such a magnet with the other pole as indicated in figure
202. The number of such lines per unit area represents the intensity of the magnetic field in that area. If two such magnetic
bars or magnets are placed side by side, the like poles will repel each other and the unlike poles will attract each other.

Figure 202 – Lines of magnetic force about a magnet
203. Magnetism is in general of two types, permanent and induced. A bar having permanent magnetism will retain its
magnetism when it is removed from the magnetizing field. A bar having induced magnetism will lose its magnetism when
removed from the magnetizing field. Whether or not a bar will retain its magnetism on removal from the magnetizing field
will depend on the strength of that field, the degree of hardness of the iron (retentivity), and also upon the amount of physical
stress applied to the bar while in the magnetizing field. The harder the iron the more permanent will be the magnetism
acquired.
204. Terrestrial magnetism. The accepted theory of terrestrial magnetism considers the earth as a huge magnet surrounded
by lines of magnetic force that connect its two magnetic poles. These magnetic poles are near, but not coincidental, with the
geographic poles of the earth. Since the north-seeking end of a compass needle is conventionally called a red pole, north pole,
or positive pole, it must therefore be attracted to a pole of opposite polarity, or to a blue pole, south pole, or negative pole.

The magnetic pole near the north geographic pole is therefore a blue pole, south pole, or negative pole; and the magnetic pole
near the south geographic pole is a red pole, north pole, or positive pole.
205. Figure 205 illustrates the earth and its surrounding magnetic field. The flux lines enter the surface of the earth at
different angles to the horizontal, at different magnetic latitudes. This angle is called the angle of magnetic dip, θ, and
increases from zero, at the magnetic equator, to 90° at the magnetic poles. The total magnetic field is generally considered as
having two components, namely H, the horizontal component, and Z, the vertical component. These components change as
the angle θ changes such that H is maximum at the magnetic equator and decreases in the direction of either pole; Z is zero at
the magnetic equator and increases in the direction of either pole.

4


Figure 205 – Terrestrial magnetism
206. Inasmuch as the magnetic poles of the earth are not coincidental with the geographic poles, it is evident that a compass
needle in line with the earth's magnetic field will not indicate true north, but magnetic north. The angular difference between
the true meridian (great circle connecting the geographic poles) and the magnetic meridian (direction of the lines of magnetic
flux) is called variation. This variation has different values at different locations on the earth. These values of magnetic
variation may be found on the compass rose of navigational charts. The variation for most given areas undergoes an annual
change, the amount of which is also noted on all charts. See figure 206.

Figure 206 – Compass rose showing variation and annual change
5


207. Ship's magnetism. A ship, while in the process of being constructed, will acquire magnetism of a permanent nature
under the extensive hammering it receives in the earth's magnetic field. After launching, the ship will lose some of this
original magnetism as a result of vibration, pounding, etc., in varying magnetic fields, and will eventually reach a more or
less stable magnetic condition. This magnetism which remains is the permanent magnetism of the ship.
208. The fact that a ship has permanent magnetism does not mean that it cannot also acquire induced magnetism when placed
in a magnetic field such as the earth's field. The amount of magnetism induced in any given piece of soft iron is dependent

upon the field intensity, the alignment of the soft iron in that field, and the physical properties and dimensions of the iron.
This induced magnetism may add to or subtract from the permanent magnetism already present in the ship, depending on
how the ship is aligned in the magnetic field. The softer the iron, the more readily it will be induced by the earth's magnetic
field and the more readily it will give up its magnetism when removed from that field.
209. The magnetism in the various structures of a ship which tends to change as a result of cruising, vibration, or aging, but
does not alter immediately so as to be properly termed induced magnetism, is called subpermanent magnetism. This
magnetism, at any instant, is recognized as part of the ship's permanent magnetism, and consequently must be corrected as
such by means of permanent magnet correctors. This subpermanent magnetism is the principal cause of deviation changes on
a magnetic compass. Subsequent reference to permanent magnetism in this text will refer to the apparent permanent
magnetism that includes the existing permanent and subpermanent magnetism at any given instant.
210. A ship, then, has a combination of permanent, subpermanent, and induced magnetism, since its metal structures are of
varying degrees of hardness. Thus, the apparent permanent magnetic condition of the ship is subject to change from
deperming, excessive shocks, welding, vibration, etc.; and the induced magnetism of the ship will vary with the strength of
the earth's magnetic field at different magnetic latitudes, and with the alignment of the ship in that field.
211. Resultant induced magnetism from earth's magnetic field. The above discussion of induced magnetism and
terrestrial magnetism leads to the following facts. A long thin rod of soft iron in a plane parallel to the earth's horizontal
magnetic field, H, will have a red (north) pole induced in the end toward the north geographic pole and a blue (south) pole
induced in the end toward the south geographic pole. This same rod in a horizontal plane but at right angles to the horizontal
earth's field would have no magnetism induced in it, because its alignment in the magnetic field is such that there will be no
tendency toward linear magnetization and the rod is of negligible cross section. Should the rod be aligned in some horizontal
direction between those headings that create maximum and zero induction, it would be induced by an amount that is a
function of the angle of alignment. If a similar rod is placed in a vertical position in northern latitudes so as to be aligned with
the vertical earth's field Z, it will have a blue (south) pole induced at the upper end and a red (north) pole induced at the lower
end. These polarities of vertical induced magnetization will be reversed in southern latitudes. The amount of horizontal or
vertical induction in such rods, or in ships whose construction is equivalent to combinations of such rods, will vary with the
intensity of H and Z, heading, and heel of the ship.

6

CHAPTER III
THEORY OF MAGNETIC COMPASS ADJUSTMENT
301. Magnetic adjustment. The magnetic compass, when used on a steel ship, must be so corrected for the ship's magnetic
conditions that its operation approximates that of a nonmagnetic ship. Ship's magnetic conditions create deviations of the
magnetic compass as well as sectors of sluggishness and unsteadiness. Deviation is defined as deflection of the card (needles)
to the right or left of the magnetic meridian. Adjustment of the compass is the arranging of magnetic and soft iron correctors
about the binnacle so that their effects are equal and opposite to the effects of the magnetic material in the ship, thus reducing
the deviations and eliminating the sectors of sluggishness and unsteadiness.
The magnetic conditions in a ship which affect a magnetic compass are permanent magnetism and induced magnetism, as
discussed in Chapter II.
302. Permanent magnetism and its effects on the compass. The total permanent magnetic field effect at the compass may
be broken into three components mutually 90° apart, as shown in figure 302a. The effect of the vertical permanent
component is the tendency to tilt the compass card and, in the event of rolling or pitching of the ship to create oscillating
deflections of the card. Oscillation effects that accompany roll are maximum on north and south compass headings, and those
that accompany pitch are maximum on east and west compass headings. The horizontal B and C components of permanent
magnetism cause varying deviations of the compass as the ship swings in heading on an even keel. Plotting these deviations
against compass heading will produce sine and cosine curves, as shown in figure 302b. These deviation curves are called
semicircular curves because they reverse direction in 180°.

Figure 302a – Components of permanent magnetic
field at the compass

Figure 302b – Permanent magnetic deviation effects

303. The permanent magnetic semicircular deviations can be illustrated by a series of simple sketches, representing a ship on
successive compass headings, as in figures 303a and 303b.
304. The ships illustrated in figures 303a and 303b are pictured on cardinal compass headings rather than on cardinal
magnetic headings, for two reasons:
(1) Deviations on compass headings are essential in order to represent sinusoidal curves that can be analyzed
mathematically. This can be visualized by noting that the ship's component magnetic fields are either in line with or

perpendicular to the compass needles only on cardinal compass headings.
(2) Such a presentation illustrates the fact that the compass card tends to float in a fixed position, in line with the
magnetic meridian. Deviations of the card to right or left (east or west) of the magnetic meridian result from the
movement of the ship and its magnetic fields about the compass card.

7


Figure 303a – Force diagrams for fore-and-aft permanent B magnetic field

Figure 303b – Force diagrams for athwartship permanent C magnetic field
305. Inasmuch as a compass deviation is caused by the existence of a force at the compass that is superimposed upon the
normal earth's directive force, H, a vector analysis is helpful in determining deviations or the strength of deviating fields. For
example, a ship as shown in figure 305 on an east magnetic heading will subject its compass to a combination of magnetic
effects; namely, the earth's horizontal field H, and the deviating field B, at right angles to the field H. The compass needle
will align itself in the resultant field which is represented by the vector sum of H and B, as shown. A similar analysis on the
ship in figure 305 will reveal that the resulting directive force at the compass would be maximum on a north heading and
minimum on a south heading, the deviations being zero for both conditions.
The magnitude of the deviation caused by the permanent B magnetic field will vary with different values of H; hence,
deviations resulting from permanent magnetic fields will vary with the magnetic latitude of the ship.

8


Figure 305 – General force diagram
306. Induced magnetism and its effects on the compass. Induced magnetism varies with the strength of the surrounding
field, the mass of metal, and the alignment of the metal in the field. Since the intensity of the earth's magnetic field varies
over the earth's surface, the induced magnetism in a ship will vary with latitude, heading, and heel of the ship.
307. With the ship on an even keel, the resultant vertical induced magnetism, if not directed through the compass itself, will
create deviations that plot as a semicircular deviation curve. This is true because the vertical induction changes magnitude

and polarity only with magnetic latitude and heel and not with heading of the ship. Therefore, as long as the ship is in the
same magnetic latitude, its vertical induced pole swinging about the compass will produce the same effect on the compass as
a permanent pole swinging about the compass. Figure 307a illustrates the vertical induced poles in the structures of a ship.

Figure 307a – Ship's vertical induced magnetism

Figure 307b – Induced magnetic deviation effects

Generally, this semicircular deviation will be a B sine curve, as shown in figure 307b, since most ships are symmetrical
about the centerline and have their compasses mounted on the centerline. The magnitude of these deviations will change with
magnetic latitude changes because the directive force and the ship's vertical induction both change with magnetic latitude.
308. The masses of horizontal soft iron that are subject to induced magnetization create characteristic deviations, as indicated
in figure 307b. The D and E deviation curves are called quadrantal curves because they reverse polarity in each of the four
quadrants.

9


309. Symmetrical arrangements of horizontal soft iron may exist about the compass in any one of the patterns illustrated in
figure 309.

Figure 309 – Symmetrical arrangements of horizontal soft iron
310. The deviation resulting from the earth's field induction of these symmetrical arrangements of horizontal soft iron are
illustrated in figure 310, showing the ship on various compass headings. The other heading effects may be similarly studied.
Such a D deviation curve is one of the curves in figure 307b. It will be noted that these D deviations are maximum on the
intercardinal headings and zero on the cardinal headings.

Figure 310 – Effects of symmetrical horizontal D induced magnetism

10

311. Asymmetrical arrangements of horizontal soft iron may exist about the compass in a pattern similar to one of those in
figure 311.

Figure 311 – Asymmetrical arrangements of horizontal soft iron
312. The deviations resulting from the earth's field induction of these asymmetrical arrangements of horizontal soft iron are
illustrated in figure 312, showing the ship on different compass headings. The other heading effects may be similarly studied.
Such an E deviation curve is one of the curves in figure 307b. It will be observed that these E deviations are maximum on
cardinal headings and zero on the intercardinal headings.

Figure 312 – Effects of asymmetrical horizontal E induced magnetism
313. The quadrantal deviations will not vary with latitude changes, because the horizontal induction varies proportionally
with the directive force, H.
314. The earth's field induction in certain other asymmetrical arrangements of horizontal soft iron creates a constant A
deviation curve. The magnetic A and E errors are of smaller magnitude than the other errors, but, when encountered, are
generally found together, since they both result from asymmetrical arrangements of horizontal soft iron. In addition to this
magnetic A error, there are constant A deviations resulting from: (1) physical misalignments of the compass, pelorus, or gyro;
(2) errors in calculating the sun's azimuth, observing time, or taking bearings.
11


315. The nature, magnitude, and polarity of all these induced effects are dependent upon the disposition of metal, the
symmetry or asymmetry of the ship, the location of the binnacle, the strength of the earth's magnetic field, and the angle of
dip.
316. Certain heeling errors, in addition to those resulting from permanent magnetism, are created by the presence of both
horizontal and vertical soft iron, which experience changing induction as the ship rolls in the earth's magnetic field. This part
of the heeling error will naturally change in magnitude with changes of magnetic latitude of the ship. Oscillation effects
accompanying roll are maximum on north and south headings, just as with the permanent magnetic heeling errors.
317. Adjustments and correctors. Since some magnetic effects remain constant for all magnetic latitudes and others vary

with changes of magnetic latitude, each individual effect should be corrected independently. Further, it is apparent that the
best method of adjustment is to use (1) permanent magnet correctors to create equal and opposite vectors of permanent
magnetic fields at the compass, and (2) soft iron correctors to assume induced magnetism, the effect of which will be equal
and opposite to the induced effects of the ship for all magnetic latitude and heading conditions. The compass binnacle
provides for the support of the compass and such correctors. Study of the binnacle in figure 317 will reveal that such
correctors are present in the form of:
(1) Vertical permanent heeling magnet in the central vertical tube,
(2) Fore-and-aft B permanent magnets in their trays,
(3) Athwartship C permanent magnets in their trays,
(4) Vertical soft iron Flinders bar in its external tube,
(5) Soft iron spheres.
The heeling magnet is the only corrector that corrects for both permanent and induced effects, and consequently must be
readjusted occasionally with radical changes in latitude of the ship. (It must be noted, however, that any movement of the
heeling magnet will require readjustment of other correctors.)

Figure 317 – Binnacle with compass and correctors
12


318. The tabular summary of "Compass Errors and Adjustments," figure 318, summarizes all the various magnetic conditions in a ship, the types of
deviation curves they create, the correctors for each effect, and headings on which each corrector is adjusted. Correctors should be applied
symmetrically under all but exceptional conditions (discussed in detail later) and as far away from the compass as possible to preserve uniformity of
magnetic fields about the compass needle array. Other details of corrector procedure are emphasized in chapter VI. Fortunately, each magnetic effect has
a slightly different characteristic curve that makes identification and correction convenient. A complete deviation curve can be analyzed for its different
components and, thus, the necessary corrections anticipated. A method for analyzing such curves is described in chapter V.

Coefficient

A

B

C

Type deviation curve

Constant.

Semicircular sin ø.

Semicircular cos ø.

D

Quadrantal sin 2ø.

E

Quadrantal cos 2ø.

Heeling

Oscillations with roll
or pitch.
Deviations with
constant list.

Compass
headings of
maximum

deviation

Causes of such errors

Corrections for such errors

Human: error in calculations..........................................
Physical: compass, gyro, pelorus alignment...................
Magnetic: asymmetrical arrangements of horizontal soft
iron ...............................................................

Check methods and calculations
Check alignments

090°
270°

Fore-and-aft component of permanent magnetic field....
Induced magnetism in asymmetrical vertical iron
forward or aft of compass ..............................................

Fore-and-aft B magnets.

000°
180°

Athwartship component of permanent magnetic field....
Induced magnetism in asymmetrical vertical iron port or
starboard of compass....................................................

Athwartship C magnets

045°
135°
225°
315°

Induced magnetism in all symmetrical arrangements of
horizontal soft iron.

Spheres on appropriate axis.
(athwartship for +D)
(fore and aft for -D)
See sketch a

000°
090°
180°
270°

Induced magnetism in all asymmetrical arrangements
of horizontal soft iron.

Spheres on appropriate axis.
(port forward, starboard aft for +E)
(starboard forward, port aft for -E)
See sketch b

Change in the horizontal component of the induced or
permanent magnetic fields at the compass due to

rolling or pitching of the ship

Heeling magnet (must be re-adjusted
for latitude changes)

Same on all.

000°
180°
090°
270°

roll
pitch

Any.

Rare arrangement of soft iron rods
090° or 270°.

Flinders bar (forward or aft)
000° or 180°.

Flinders bar (port or starboard)

Deviation = A + B sin ø + C cos ø + D sin 2 ø + E cos 2 ø (ø = compass heading)

Figure 318 – Summary of Compass Errors and Adjustments
13

Magnetic or compass
headings on which to apply
correctors

045°, 135°, 225°, or 315°.

000°, 090°, 180°, or 270°.

090° or 270° with dip
needle.
000° or 180° while rolling.


319. Compass operation. Figure 319 illustrates a point about compass operation. Not only is an uncorrected compass subject
to large deviations, but there will be sectors in which the compass may sluggishly turn with the ship and other sectors in
which the compass is too unsteady to use. These performances may be appreciated by visualizing a ship with deviations as
shown in figure 319, as it swings from west through north toward east. Throughout this easterly swing the compass deviation
is growing more easterly; and, whenever steering in this sector, the compass card sluggishly tries to follow the ship.
Similarly, there is an unsteady sector from east through south to west. These sluggish and unsteady conditions are always
characterized by the positive and negative slopes in a deviation curve. These conditions may also be associated with the
maximum and minimum directive force acting on the compass (see article 305). It will be observed that the maximum
deviation occurs at the point of average directive force and that the zero deviations occur at the points of maximum and
minimum directive force.

Figure 319 – Uncompensated deviation curve
320. Correction of compass errors is generally achieved by applying correctors so as to reduce the deviations of the compass
for all headings of the ship. Correction could be achieved, however, by applying correctors so as to equalize the directive
forces across the compass position for all headings of the ship. The deviation method is more generally used because it
utilizes the compass itself to indicate results, rather than some additional instrument for measuring the intensity of magnetic
fields.

321. Occasionally, the permanent magnetic effects at the location of the compass are so large that they overcome the earth's
directive force, H. This condition will not only create sluggish and unsteady sectors, but may even freeze the compass to one
reading or to one quadrant, regardless of the heading of the ship. Should the compass be so frozen, the polarity of the
magnetism which must be attracting the compass needles is indicated; hence, correction may be effected simply by the
application of permanent magnet correctors in suitable quantity to neutralize this magnetism. Whenever such adjustments are
made, it would be well to have the ship placed on a heading such that the unfreezing of the compass needles will be
immediately evident. For example, a ship whose compass is frozen to a north reading would require fore-and-aft B corrector
magnets with the red ends forward in order to neutralize the existing blue pole that attracted the compass. If made on an east
heading, such an adjustment would be practically complete when the compass card was freed so as to indicate an east
heading.
322. Listed below are several reasons for correcting the errors of the magnetic compass:
(1) It is easier to use a magnetic compass if the deviations are small.
(2) Although a common belief is that it does not matter what the deviations are, as long as they are known, this is in
error inasmuch as conditions of sluggishness and unsteadiness accompany large deviations and consequently make
the compass operationally unsatisfactory. This is the result of unequal directive forces on the compass as the ship
swings in heading.
(3) Furthermore, even though the deviations are known, if they are large they will be subject to appreciable change with
heel and latitude changes of the ship.
323. Subsequent chapters will deal with the methods of bringing a ship to the desired heading, and the methods of isolating
deviation effects and of minimizing interaction effects between correctors. Once properly adjusted, the magnetic compass
deviations should remain constant until there is some change in the magnetic condition of the vessel resulting from magnetic
treatment, shock from gunfire, vibration, repair, or structural changes. Frequently, the movement of nearby guns, doors, gyro
repeaters, or cargo affects the compass greatly.

14


CHAPTER IV
PRACTICAL PROCEDURES FOR MAGNETIC COMPASS ADJUSTMENT
NOTE: If the adjuster is not familiar with the theory of magnetic effects, the methods of analyzing deviation curves, and the

methods of placing a ship on any desired heading, it is recommended to review Chapters II, V and VII, respectively, before
beginning adjustment.
401. Dockside tests and adjustments. Chapter I, "Procedures for Magnetic Compass Adjustment" is, in general, selfexplanatory, and brings to the attention of the adjuster many physical checks which are desirable before beginning an
adjustment. The theoretical adjustment is based on the premise that all the physical arrangements are perfect, and much time
and trouble will be saved while at sea if these checks are made before attempting the actual magnet and soft iron corrector
adjustments. A few of these checks are amplified below.
402. Should the compass have a small bubble, compass fluid may be added by means of the filling plug on the side of the
compass bowl. If an appreciable amount of compass liquid has leaked out, a careful check should be made on the condition
of the sealing gasket and filling plug. U.S. Navy compass liquid may be a mixture of 45% grain alcohol and 55% distilled
water, or a kerosene-type fluid. These fluids are NOT interchangeable.
403. The compass should be removed from the ship and taken to some place free from all magnetic influences except the
earth's magnetic field for tests of moment and sensibility. These tests involve measurements of the time of vibration and the
ability of the compass card to return to a consistent reading after deflection. These tests will indicate the condition of the
pivot, jewel, and magnetic strength of the compass needles. (See NAVSEA 3120/3 for such test data on standard Navy
compass equipment.)
404. A careful check should be made on the spheres and Flinders bar for residual magnetism. Move the spheres as close to
the compass as possible and slowly rotate each sphere separately. Any appreciable deflection (2° or more) of the compass
needles resulting from this rotation indicates residual magnetism in the spheres. This test may be made with the ship on any
steady heading. The Flinders bar magnetization check is preferably made with the ship on steady east or west compass
heading. To make this check: (a) note the compass reading with the Flinders bar in the holder; (b) invert the Flinders bar in
the holder and again note the compass reading. Any appreciable difference (2° or more) between these observed readings
indicates residual magnetism in the Flinders bar. Spheres or Flinders bars that show signs of such residual magnetism should
be annealed, i.e. heated to a dull red and allowed to cool slowly.
405. Correct alignment of the lubber's line of the compass, gyro repeater, and pelorus with the fore-and-aft line of the ship is
of major importance. Such a misalignment will produce a constant A error in the curve of deviations. All of these instruments
may be aligned correctly with the fore-and-aft line of the ship by using the azimuth circle and a metal tape measure. Should
the instrument be located on the centerline of the ship, a sight is taken on a mast or other object on the centerline. In the case
of an instrument off the centerline, a metal tape measure is used to measure the distance from the centerline of the ship to the
center of the instrument. A similar measurement from the centerline is made forward or abaft the subject instrument and
reference marks are placed on the deck. Sights are then taken on these marks.

Standard compasses should always be aligned so that the lubber's line of the compass is parallel to the fore-and-aft line of
the ship. Steering compasses may occasionally be deliberately misaligned in order to correct for any magnetic A error
present, as discussed in article 411.
406. In addition to the physical checks listed in Chapter I, there are other precautions to be observed in order to assure
continued satisfactory compass operation. These precautions are mentioned to bring to the attention of the adjuster certain
conditions that might disturb compass operation. They are listed in Chapter I and are discussed further in Chapter X.
Expeditious compass adjustment is dependent upon the application of the various correctors in an optimum sequence so as
to achieve the final adjustment with a minimum number of steps. Certain adjustments may be made conveniently at dockside
so as to simplify the adjustment procedures at sea.
407. Inasmuch as the Flinders bar is subject to induction from several of the other correctors and, since its adjustment is not
dependent on any single observation, this adjustment is logically made first. This adjustment is made by one of the following
methods:
(1) Deviation data obtained at two different magnetic latitudes may be utilized to calculate the proper length of Flinders
bar for any particular compass location. Details of the acquisition of such data and the calculations involved are
presented in articles 605 to 609, inclusive.
15


(2) If the above method is impractical, the Flinders bar length will have to be set approximately by:
(a) Using an empirical amount of Flinders bar that has been found correct for other ships of similar structure.
(b) Studying the arrangement of masts, stacks, and other vertical structures and estimating the Flinders bar length
required.
If these methods are not suitable, the Flinders bar should be omitted until data is acquired.
The iron sections of Flinders bar should be continuous and at the top of the tube with the longest section at the top.
Wooden spacers are used at the bottom of the tube to achieve such spacing.
408. Having adjusted the length of Flinders bar, place the spheres on the bracket arms at the best approximate position. If the
compass has been adjusted previously, place the spheres at the best position as indicated by the previous deviation table. In
the event the compass has never been adjusted, place the spheres at midposition on the bracket arms.
409. The next adjustment is the positioning of the heeling magnet by means of a properly balanced dip needle, as discussed
in Chapter XI.

410. These three adjustments at dockside - Flinders bar, spheres, and heeling magnet - will properly establish the conditions
of mutual induction and shielding on the compass, such that a minimum of procedures at sea will complete the adjustment.
411. Expected errors. Figure 318, "Summary of Compass Errors and Adjustment", lists six different coefficients or types of
deviation errors with their causes and corresponding correctors. A discussion of these coefficients follows:
The A error is more generally caused by the miscalculation of azimuths or by physical misalignments, rather than magnetic
effects of asymmetrical arrangements of horizontal soft iron. Thus, if the physical alignments are checked at dockside, and if
care is exercised in making all calculations, the A error will be insignificant. Where an azimuth or bearing circle is used on a
standard compass to determine deviations, any observed A error will be solely magnetic A error. This results from the fact
that such readings are taken on the face of the compass card itself rather than at the lubber's line of the compass. On a
steering compass where deviations are obtained by a comparison of the compass lubber's line reading with the ship's
magnetic heading as determined by pelorus or gyro, any observed A error may be a combination of magnetic A and
mechanical A (misalignment). These facts explain the procedure wherein only mechanical A is corrected on the standard
compass by realignment of the binnacle, and both mechanical A and magnetic A errors are corrected on the steering compass
by realignment of the binnacle (see article 405). On the standard compass, the mechanical A error may be isolated from the
magnetic A error by making the following observations simultaneously:
(1) Record a curve of deviations by using an azimuth (or bearing) circle. An A error found will be solely magnetic A.
(2) Record a curve of deviations by comparison of the compass lubber's line reading with the ship's magnetic heading as
determined by pelorus or by gyro. Any A error found will be a combination of mechanical A and magnetic A.
The mechanical A on the standard compass is then found by subtracting the A found in the first instance from the total A
found in the second instance, and is corrected by rotating the binnacle in the proper direction by that amount. It is neither
convenient nor necessary to isolate the two types of A on the steering compass and all A found by using the pelorus or gyro
may be removed by rotating the binnacle in the proper direction by that amount.
The B error results from two different causes, namely: the fore-and-aft permanent magnetic field across the compass, and a
resultant asymmetrical vertical induced effect forward or aft of the compass. The former is corrected by the use of fore-andaft B magnets, and the latter is corrected by the use of the Flinders bar forward or aft of the compass. Inasmuch as the
Flinders bar setting has been made at dockside, any B error remaining is corrected by the use of fore-and-aft B magnets.
The C error has two causes, namely: the athwartship permanent magnetic field across the compass, and a resultant
asymmetrical vertical induced effect athwartship of the compass. The former is corrected by the use of athwartship C
magnets, and the latter by the use of the Flinders bar to port or starboard of the compass; but, inasmuch as this vertical
induced effect is very rare, the C error is corrected by athwartship C magnets only.
The D error is due only to induction in the symmetrical arrangements of horizontal soft iron, and requires correction by

spheres, generally athwartship of the compass.
The existence of E error of appreciable magnitude is rare, since it is caused by induction in the asymmetrical arrangements
of horizontal soft iron. When this error is appreciable it may be corrected by slewing the spheres, as described in Chapter VI.
As has been stated previously, the heeling error is most practically adjusted at dockside with a balanced dip needle. (See
Chapter XI.)
412. A summary of the above discussion reveals that certain errors are rare, and others have been corrected by adjustments at
dockside. Therefore, for most ships, there remain only three errors to be corrected at sea, namely the B, C, and D errors.
These are corrected by the use of fore-and-aft B magnets, athwartship C magnets, and quadrantal spheres respectively.

16


413. Study of adjustment procedure. Inspection of the general B, C, and D combination of errors, pictured in figure 413 will
reveal that there is a definite isolation of the deviation effects on cardinal compass headings.

Figure 413 – B, C, and D deviation effects
For example, on 090° or 270° compass headings, the only deviation which is effective is that due to B. This isolation, and
the fact that the B effect is greatest on these two headings, make these headings convenient for B correction. Correction of the
B deviation on a 090° heading will correct the B deviation on the 270° heading by the same amount but in the opposite
direction and naturally, it will not change the deviations on the 000° and 180° headings, except where B errors are large.
However, the total deviation on all the intercardinal headings will be shifted in the same direction as the adjacent 090° or
270° deviation correction, but only by seven-tenths (0.7) of that amount, since the sine of 45° equals 0.707.
The same convenient isolation of effects and corrections of C error will also change the deviations on all the intercardinal
headings by the seven-tenths rule, as before. It will now be observed that only after correcting the B and C errors on the
cardinal headings, and consequently their proportional values of the total curve on the intercardinal headings, can the D error
be observed separately on any of the intercardinal headings. The D error may then be corrected by use of the spheres on any
intercardinal heading. Correcting D error will, as a rule, change the deviations on the intercardinal headings only and not on
the cardinal headings. Only when the D error is excessive, the spheres are magnetized, or the permanent magnet correctors
are so close as to create excessive induction in the spheres will there be a change in the deviations on cardinal headings as a
result of sphere adjustments. Although sphere correction does not generally correct deviations on cardinal headings, it does

improve the stability of the compass on these headings.
414. If it were not for the occasional A or E errors which exist, the above procedure of adjustment would be quite sufficient,
i.e., adjust observed deviations to zero on two adjacent cardinal headings and then on the intermediate intercardinal heading.
However, figure 414, showing a combination of A and B errors, will illustrate why adjusting procedure must include
correcting deviations on more than the three essential headings.
If the assumption were made that no A error existed in the curve illustrated in figure 414, and the total deviation of 6°E on
the 090° heading were corrected with B magnets, the error on the 270° heading would be 4°E due to B overcorrection. If then,
this 4°E error were taken out on the 270° heading, the error on the 090° heading would then be 4°E due to B undercorrection.

Figure 414 – A and B deviation effects

17


The proper method of eliminating this to-and-fro procedure, and also correcting the B error of the ship to the best possible
flat curve, would be to split this 4°E difference, leaving 2°E deviation on each opposite heading. This would, in effect correct
the B error, leaving only the A error of 2°E which must be corrected by other means. It is for this reason that, (1) splitting is
done between the errors noted on opposite headings, and (2) good adjustments entail checking on all headings rather than on
the fundamental three.
415. Before anything further is said about adjustment procedures, it is suggested that care be exercised to avoid moving the
wrong corrector. Not only will such practice be a waste of time but it will also upset all previous adjustments and
calculations. Throughout an adjustment, special care should be taken to pair off spare magnets so that the resultant field about
them will be negligible. To make doubly sure that the compass is not affected by stray fields from them, they should be kept
at an appropriate distance until one or more is actually to be inserted into the binnacle.
416. Adjustment procedures at sea. Before proceeding with the adjustment at sea, the following precautions should be
observed:
(1) Secure all effective magnetic gear in the normal seagoing position.
(2) Make sure the degaussing coils are secured, using the reversal sequence, if necessary.
The adjustments are made with the ship on an even keel, swinging from heading to heading slowly, and after steadying on
each heading for at least 2 minutes to avoid Gaussin error (article 1003). Chapter VII discusses methods of placing a ship on

the desired heading.
417. Most adjustments can be made by trial and error, or by routine procedure such as the one presented in Chapter I.
However, it is more desirable to follow some analytical procedure whereby the adjuster is always aware of the magnitude of
the errors on all headings as a result of his movement of the different correctors. Two such methods are presented:
(1) A complete deviation curve can be taken for any given condition, and an estimate made of all the approximate
coefficients. See Chapter V for methods of making such estimates. From this estimate, the approximate coefficients are
established and the appropriate corrections are made with reasonable accuracy on a minimum number of headings. If the
original deviation curve has deviations greater than 20°, rough adjustments should be made on two adjacent cardinal
headings before recording curve data for such analysis. The mechanics of applying correctors are presented in figure
103. A method of tabulating the anticipated deviations after each correction is illustrated in figure 417a. The deviation
curve used for illustration is the one that is analyzed in Chapter V. Analysis revealed these coefficients:
A = 1.0°E

Heading by
compass

000°
045°
090°
135°
180°
225°
270°
315°

B = 12.0°E

Original
deviation
curve

10.5°E
20.0°E
11.5°E
1.2°W
5.5°W
8.0°W
12.5° W
6.8°W

Anticipated
curve after
first correcting
A = 1.0°E
9.5°E
19.0°E
10.5°E
2.2°W
6.5°W
9.0°W
13.5°W
7.8°W

C = 8.0°E

Anticipated
curve after
next
correcting
B = 12.0°E
9.5°E

10.6°E
1.5°W
10.6°W
6.5°W
0.6°W
1.5°W
0.6°E

D = 5.0°E

Anticipated
curve after
next
correcting
C = 8.0°E
1.5°E
5.0°E
1.5°W
5.0°W
1.5°E
5.0°E
1.5°W
5.0°W

E = 1.5°E

Anticipated
curve after
next
correcting

D = 5.0°E
1.5°E
0.0°
1.5°W
0.0°
1.5°E
0.0°
1.5°W
0.0°

Figure 417a – Tabulating anticipated deviations - Analysis method.

18

Anticipated
curve after
next
correcting
E = 1.5°E
0.0°
0.0°
0.0°
0.0°
0.0°
0.0°
0.0°
0.0°


(2) More often it is desirable to begin adjustment immediately, eliminating the original swing for deviations and the

estimate or approximate coefficients. In this case the above problem would be solved by tabulating data and anticipating
deviation changes as the corrections are made. Figure 417b illustrates such procedure. It will be noted that a new column
of values is started after each change is made. This method of tabulation enables the adjuster to calculate the new
residual deviations each time a corrector is changed, so that a record of deviations is available at all times during the
swing. Arrows are used to indicate where each change is made.

Heading by
compass

Original
deviation
curve

Anticipated
curve after
first correcting
A = 1.0°E

Anticipated
curve after
next
correcting
B = 12.0°E

Anticipated
curve after
next
correcting
C = 8.0°E

Anticipated
curve after
next
correcting
D = 5.0°E

000°
045°
090°
135°
180°
225°
270°
315°

...
...
11.5°E è
…
...
...
...
...

10.5°E è
...
0.0°
9.2°W
5.5°W
0.0°

1.0°W
1.2°E

2.5°E
6.4°E è
0.0°
3.6°W
2.5°E
5.6°E
1.0°W
4.4°W

2.5°E
1.4°E è
0.0°
1.4°E
2.5°E
0.6°E
1.0°W
0.6°E

1.5°E
0.4°E
1.0°W è
0.4°E
1.5°E
0.4°W
2.0°W
0.4°W

Anticipated
curve after
next
correcting
E = 1.5°E
0.0°
0.4°E
0.5°E
0.4°E
0.0°
0.4°W
0.5°W
0.4°W

Figure 417b – Tabulating anticipated deviations - One-swing method.
Since the B error is generally greatest, it is corrected first; hence, on a 090° heading the 11.5°E deviation is corrected to
approximately zero by using fore-and-aft B magnets. A lot of time need not be spent trying to reduce this deviation to exactly
zero since the B coefficient may not be exactly 11.5°E, and some splitting might be desirable later. After correcting on the
090° heading, the swing would then be continued to 135° where a 9.2°W error would be observed. This deviation is recorded,
but no correction is made because the quadrant error is best corrected after the deviations on all four cardinal headings have
been corrected. The deviation on the 180° heading would be observed as 5.5°W. Since this deviation is not too large and
splitting may be necessary later, it need not be corrected at this time. Continuing the swing to 225° a 0.0° deviation would be
observed and recorded. On the 270° heading the observed error would be 1.0°W, which is compared with 0.0° deviation on
the opposite 090° heading. This could be split, leaving 0.5°W deviation on both 090° and 270°, but since this is so small it
may be left uncorrected. On 315° the observed deviation would be 1.2°E. At 000°, a deviation of 10.5°E would be observed
and compared with 5.5°W on 180°. Analysis of the deviations on 000° and 180° headings reveals an 8.0°E, C error, which
should then be corrected with athwartship C magnets leaving 2.5°E deviation on both the 000° and 180° headings. All the
deviations in column two are now recalculated on the basis of such an adjustment at 000° heading and entered in column
three. Continuing the swing, the deviation on 045° would then be noted as 6.4° E. Knowing the deviations on all intercardinal
headings, it is now possible to estimate the approximate coefficient D. D is 5.0°E so the 6.4°E deviation on 045° is corrected

to 10.4°E and new anticipated values are recorded in another column. This anticipates a fairly good curve, an estimate of
which reveals, in addition to the B of 0.5°E which was not considered large enough to warrant correction, an A of 1.0°E and
an E of 1.5°E. These A and E errors may or may not be corrected, as practical. If they are corrected, the subsequent steps
would be as indicated in the last two columns. It will be noted that the ship has made only one swing, all corrections have
been made, and some idea of the expected curve is available.
418. Deviation curves. The last step, after completion of either of the above methods of adjustment, is to secure all
correctors in position and to swing for residual deviations. These residual deviations are for undegaussed conditions of the
ship, which should be recorded together with details of corrector positions on the standard Navy Form NAVSEA 3120/4 and
in the Magnetic Compass Record, NAVSEA 3120/3. Article 901 discusses the purposes of the various NAVSEA Record
Forms more fully. Navy Form NAVSEA 3120/4 is complete and desirable in the interest of improved Flinders bar correction
and shielding conditions. Figure 418 illustrates both sides of form NAVSEA 3120/4 with proper instructions and sample
deviation and Flinders bar data. Should the ship be equipped with degaussing coils, a swing for residual deviations under
degaussed conditions should also be made and data recorded on NAVSEA 3120/4.
On these swings extreme care should be exercised in taking bearings or azimuths and in steadying down on each heading
since this swing is the basis of standard data for that particular compass. If there are any peculiar changeable errors, such as
movable guns, listing of the ship, or anticipated decay from deperming, which would effect the reliability of the compass,
they should also be noted on the deviation card at this time. Chapter X discusses these many sources of error in detail. If the
19


Flinders bar adjustment is not based on accurate data, as with a new ship, it would be well to exercise particular care in
recording the conventional Daily Compass Log data during the first cruise on which a considerable change of magnetic
latitude occurs.

Figure 418 – Deviation table - NAVSEA 3120/4
419. In order to have a reliable and up-to-date deviation card at all times it is suggested that the ship be swung to check
compass deviations and to make readjustments, if necessary, after:
(1) Radical changes in magnetic latitude.
(2) Deperming. (Delay adjustment several days, if possible, after treatment.)
(3) Structural changes.

(4) Long cruises or docking on the same heading such that the permanent magnetic condition of the vessels has
changed.
(5) Magnetic equipment near the binnacle has been altered.
(6) Reaching the magnetic equator, in order to acquire Flinders bar data. (See Chapter VI.)
(7) At least once yearly, to account for magnetic decay, etc.
(8) Appreciable change of heeling magnet position if Flinders bar is present.
(9) Readjustment of any corrector.
(10) Change of magnetic cargo.
(11) Commissioning.
With such reasonable care, the compass should be a reliable instrument requiring little attention.
20


CHAPTER V
TYPICAL DEVIATION CURVE AND THE ESTIMATION OF APPROXIMATE
COEFFICIENTS
501. Simple analysis. The data for the deviation curve illustrated in figure 501 is as follows:
Ship's compass heading
Deviation
N (000°) .......................................................................................... 10.5°E
NE (045°) ........................................................................................ 20.0°E
E (090°) ........................................................................................... 11.5°E
SE (135°) ........................................................................................ 1.2°W
S (180°) ........................................................................................... 5.5°W
SW (225°) ....................................................................................... 8.0°W
W (270°) ......................................................................................... 12.5°W
NW (315°) ...................................................................................... 6.8°W

Figure 501 – Typical deviation curve and its individual components
Since A is the coefficient of constant deviation, its approximate value is obtained from the above data by estimating the

mean of the algebraic sum of all the deviations. Throughout these computations the sign of east deviation is considered plus,
and west deviation is considered minus.
8A = + 10.5° + 20.0° + 11.5° – 1.2° – 5.5° – 8.0° – 12.5° – 6.8°
8A = + 42.0° – 34.0°
8A = + 8.0°
A = + 1.0° (1.0°E)
Since B is the coefficient of semicircular sine deviation, its value is maximum, but of opposite polarity, on 090° and 270°
headings. The approximate B coefficient is estimated by taking the mean of the deviations at 090° and 270° with the sign at
270° reversed.
2B = + 11.5° (+12.5°)
2B = + 24.0°
B = + 12.0° (12.0°E)

21


Similarly, since C is the coefficient of semicircular cosine deviation, its value is maximum, but of opposite polarity, on
000° and 180° headings; and the approximate C coefficient is estimated by taking the mean of the deviations at 000° and
180° with the sign at 180° reversed.
2C = + 10.5° + (+5.5°)
2C = + 16.0°
C = + 8.0° (8.0° E)
D is the coefficient of quadrantal sine deviation having maximum, but alternately opposite, polarity on the intercardinal
headings. Hence, the approximate D coefficient is estimated by taking the mean of the four intercardinal deviations with the
signs at 135° and 315° reversed.
40 = (+20.0°) + (+1.2°) + (–8.0°) + (+6.8°)
40 = + 20.0°
D = + 5.0° (5.0°E)
E is the coefficient of quadrantal cosine deviation having maximum, but alternately opposite, polarity on the cardinal
headings. Therefore, the approximate E coefficient is estimated by taking the mean of the four cardinal deviations with the

signs at 090° and 270° reversed.
4E = (+10.5°) + (–11.5°) + (–5.5°) + (+12.5°)
4E = + 6.0°
E = + 1.5° (1.5°E)
These approximate coefficients are estimated from deviations on compass headings rather than on magnetic headings. The
arithmetic solution of such coefficients will automatically assign the proper polarity to each coefficient. Summarizing the
above we find the approximate coefficients of the given deviation curve to be:
A = 1.0°E
B = 12.0°E
C = 8.0°E
D = 5.0°E
E = 1.5°E
Each of these coefficients represents a component of deviation that can be plotted as shown in figure 501. The polarity of
each component in the first quadrant must agree with the polarity of the coefficient. A check on the components in figure 501
will reveal that their summation equals the original curve. This method of analysis is accurate only when the deviations are
less than 20°. The mathematical expression for the deviation on any heading, using the approximate coefficients, is:
Deviation = A + B sin ø + C cos ø + D sin 2ø + E cos 2ø
(where ø represents compass heading)
The directions given above for calculating coefficients A and B are not based upon accepted theoretical methods of
estimation. Some cases may exist where appreciable differences may occur in the coefficients as calculated by the above
method and the accepted theoretical method. The proper calculation of coefficients B and C is as follows:
Letting D1, D2, .…., D8 be the eight deviation data, then

B=

√2
1
(D3 – D7)
(D2 + D4 – D6 – D8) +
4

8

C=

√2
1
(D2 – D4 – D6 + D8) +
(D1 – D5)
8
4

22


Substituting deviation data algebraically, east being plus and west minus,
B=

√2
1
(20.0 – 1.2 + 8.0 + 6.8) +
(11.5 + 12.5)
8
4

B = +12

C=

√2
1

(20.0 + 1.2 + 8.0 – 6.8) +
(10.5 + 5.5)
8
4

C = +8
502. Reasons for analysis. This method of estimating approximate coefficients is convenient for:
(1) Analyzing an original deviation curve in order to anticipate necessary corrections.
(2) Analyzing a final deviation curve for the determination of additional refinements.
(3) Simplifying the actual adjustment procedure by anticipating effects of certain corrector changes on the deviations at
all other headings.
503. Approximate and exact coefficients. It is emphasized that the above estimations are for the approximate coefficients
and not for exact coefficients. Approximate coefficients are in terms of angular deviations that are caused by certain magnetic
forces, and as stated before, some of these deviations are subject to change with changes in the directive force, H. The exact
coefficients are expressions of magnetic forces, dealing with: (a) arrangements of soft iron, (b) components of permanent
magnetic fields, (c) components of the earth's magnetic field, and (d) the shielding factor λ. Thus, the exact coefficients are
expressions of magnetic force which produce the deviations expressed by the approximate coefficients. The exact coefficients
are for mathematical considerations, while the approximate coefficients are more practical for adjustment purposes. For this
reason, the exact coefficients and the associated mathematics are not expanded further in this text.
504. Compass heading and magnetic heading. When deviations are large, there is an appreciable difference in the deviation
curve if it is plotted on cross-section paper against compass headings or against magnetic headings of the ship. Not only is
there a difference in the shape of the curves, but if only one curve is available, navigators will find it difficult in applying
deviations when converting from magnetic heading to compass heading, and vice versa. When deviations are small no
conversion is necessary. Figure 504 illustrates the differences mentioned above by presenting the deviation values used in
figure 501 as plotted against magnetic headings as well as against compass headings.

Figure 504 – Comparison of deviation curves (magnetic heading vs. compass heading).

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CHAPTER VI
CORRECTOR EFFECTS-INTERACTIONS BETWEEN CORRECTORS
601. Until now the principles of compass adjustment have been considered from a qualitative point of view. In general, this is
quite sufficient since the correctors need merely be moved until the desired amount of correction is obtained. However, it is
often valuable to know the quantitative effects of different correctors as well as their qualitative effects. Furthermore, as has
been stated previously, all the correctors are not completely independent of each other. Interaction results from the proximity
of the permanent magnet correctors to the soft iron correctors, with appreciable induction effects in the latter. Consequently
any shift in the relative position of the various correctors will change their interaction effects as well as their separate
correction effects. Additional inductions exist in the soft iron correctors from the magnetic needles of the compass itself. The
adjuster should therefore be familiar with the nature of these interactions so as to evolve the best methods of adjustment.
602. Quandrantal sphere correction. Figure 602 presents the approximate quadrantal correction available with different
sizes of spheres, at various positions on the sphere brackets, and with different magnetic moment compasses. These
quadrantal corrections apply whether the spheres are used as D, E, or combination D and E correctors. Quadrantal correction
from spheres is due partially to the earth's field induction and partially to compass needle induction. Since compass needle
induction does not change with magnetic latitude but earth's field induction does, the sphere correction is not constant for all
magnetic latitudes. A reduction in the percentage of needle induction in the spheres to the earth's field induction in the
spheres will improve the constancy of sphere correction over all magnetic latitudes. Such a reduction in the percentage of
needle induction may be obtained by:
(1) Utilizing a low magnetic moment compass (article 613).
(2) Utilizing special spheroidal-shaped correctors, placed with their major axes perpendicular to their axis of position.
(3) Using larger spheres farther away from the compass.

Figure 602
603. Slewing of spheres. Figure 603a is a convenient chart of determining the proper slewed position for spheres. The total
values of the D and E quadrantal coefficients are used on the chart to locate a point of intersection. This point directly locates
the angle and direction of slew for the spheres on the illustrated binnacle. This point will also indicate, on the radial scale, the
resultant amount of quadrantal correction required from the spheres in the new slewed position to correct for both D and E
coefficients. The total D and E coefficients may be calculated by an analysis of deviations on the uncorrected binnacle, or by
summarizing the uncorrected coefficients with those already corrected. The data in figures 602 and 603b will be useful in

either procedure.

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